We are conducting a large family study of obsessive-compulsive disorder (OCD), completing thorough psychiatric examinations of 100 OCD probands and their first-degree relatives, and 100 control probands and their first-degree relatives. The specific aims of the study are to determine if the rates of OCD, obsessive-compulsive behaviors, tic disorders, and other psychiatric conditions are higher in the relatives of the OCD probands; to compare familial and sporadic (non-familial) OCD probands with respect to clinical features; to clarify the phenotypic spectrum of conditions that may be transmitted in the OCD families; and to use segregation analysis to investigate the mode of inheritance of OCD. Probands are being randomly selected from five treatment sites. Controls, matched to the OCD cases on age, sex, and race, are being obtained by random digit dialing. We anticipate that the results of the study will indicate that OCD is familial, and we hypothesize that the condition has an underlying genetic basis. Therefore, we are collecting blood samples from members of the OCD families, and extracting DNA and freezing lymphocytes, with the purpose of conducting genetic studies in the future. To date, we have identified 80 OCD and 78 control families who have agreed to participate in the study, and a total of 508 subjects have been examined. Blood samples have been collected from 177 members of the OCD families and are stored in two participating laboratories.